The present invention relates to a method and a device for mounting and withdrawing an plasma torch relative to a installation operating under conditions of high pressure and temperature.
Plasma torches of the type having a non-transferred arc are employed in a number of installations or apparatus whose operation involves pressure and temperature conditions which preclude any direct intervention for mounting or withdrawing the torch. Under these conditions, the mounting or the withdrawal of the plasma torch requires a change in the conditions of operation of the installation or apparatus, in particular a temporary reduction in the pressure and possibly temperature operating conditions, which are detrimental from every point of view.
As the industrial development of the technique of plasma torches is generally relatively recent, there are at the present time few devices for operating these torches in industrial apparatus. As an example which characterizes an installation in which plasma torches are employed, there may be mentioned blast furnaces, and it is with respect to such an application that the invention will be described.
Plasma torches are employed in blast furnaces for superheating the hot blast coming from the cowpers before its injection into the apparatus. The injection of the hot blast is effected by means of a pipe or blast pipe to which there is connected by one end an inclined sleeve, for example as described in U.S. Pat. No. 4,670,048 of the applicant, and the nose of the torch is inserted in the opposite end of this sleeve. This sleeve must be as short as possible so as to reduce heat losses.
For the purpose of mounting in position and withdrawing the torch, the blast flow in the blast pipe must be reduced to a sufficently low pressure, on the order of about 400 mb or less and, owing to the temperature and the slight flow of hot gas, the operation is carried out under difficult conditions and results in production losses which are proportional to the duration of the change in the operating conditions of the blast furnace.
Various solutions have been envisaged for solving this problem by the use of one or more valves for isolating the nose of the torch from the blast pipe. In a first solution, a volume of the blast pipe was isolated by means of two valves disposed respectively on the upstream side and the downstream side of the sleeve receiving the nose of the torch so as to be able to put this volume under atmospheric pressure for carrying out the required operations. This arrangement permits operating the valves under acceptable conditions of .DELTA.P. However, it presents an important drawback in that it requires a major modification of the pipes of the blast furnace, whose large diameter requires the use of valves of large size resulting in high costs.
In another solution, a valve was interposed in a longer sleeve immediately on the downstream side of the nose of the torch, whereby it is possible to employ a valve having dimensions which are smaller than the foregoing dimensions. But this arrangement requires increasing the length of the sleeve, which increases the heat losses at the connecting interface and modifies the mixing conditions. Furthermore, in this solution, the valve and the refractory lining of the sleeve are partly exposed to the thermal flow of the plasma and therefore require an effective cooling by means of expensive cooling devices.
In a third arrangement, a short sleeve having just the length of the nose of the torch was provided and a valve was interposed mid-way along its length. The heat losses were in this way avoided and the valve was shielded from the harmful effects of the plasma jet. On the other hand, the valve operates under bad conditions as concerns .DELTA.P, having for result deformations which hinder its operation, and leakages. Lastly, this third arrangement prevents closing the valve when the torch is in position and leakages of hot gas occur when the torch is withdrawn, before closing the valve.